The present invention relates to magnetic tape drives useful in storing data in computer systems and, more particularly, to the control of data transfer rates between a magnetic tape cartridge drive and a host computer.
In magnetic tape cartridge drives, such as the quarter-inch cartridge drives, the drives are designed to move a magnetic tape past the magnetic heads at predetermined speeds and to lay down, or write, or to receive, or read, the data signals at a predetermined density on the magnetic tape. For example, in a QIC-525 standard drive the tape is driven at 120 inches per second and data is written onto, or read from, the tape with a density of 20,000 flux changes per inch.
These magnetic tape cartridge drives have a buffer which acts intermediate storage unit between the magnetic tape and the host device, the computer. The buffer is useful because typically the data transfer rates of the computer and the drive do not match.
Present magnetic tape cartridge drives handle the problem of disparate transfer rates between the drive and the host computer by interruptions, i.e., stopping the drive until the slower computer can catch up. During a read operation, for example, the faster drive may quickly fill the buffer as the host computer accepts the data from the buffer. At that point the drive is stopped until the computer can empty the buffer before the drive begins again transferring data from the tape into the buffer. During a write operation, the faster drive may completely empty the buffer as the computer feeds data into the buffer for writing onto the magnetic tape. When the buffer is filled again, the drive is started once more.
This stop-and-start activity is hard upon the mechanical parts of the drive. Errors may occur in the transfer of the data and the drive may even break down.